The reason you doubt Christianity's Belief A is because you hold unprovable Belief B. Every doubt, therefore, is based on a leap of faith. Some people say, "I don't believe in Christianity because I can't accept the existence of moral absolutes. Everyone should determine moral truth for him-or herself." Is that a statement they can prove to someone who doesn't share it? No, it is a leap of faith, a deep belief that individual rights operate not only in the political sphere but also in the moral. There is no empirical proof for such a position. So the doubt (of moral absolutes) is a leap. 


- The Reason for God, Tim Keller, locations 221-31